When Dean tries to enter the diner (maybe he’s one of the mysterious 6 am crowd who get up early on Saturdays?), Luke forbids him, and they end up in a physical altercation in the street. It is obvious that Luke has never liked Dean – if you look back at all their earlier interactions, Luke always had an air of underlying hostility just waiting to bubble over. He is protective of Rory, and genuinely doesn’t believe Dean is good enough for her.
The fight between them proves that Lorelai was right when she warned Dean from the beginning that he would not be safe in Stars Hollow if he ever hurt Rory, as the whole town loves her.